1. Field of the Invention
This invention relates to liquid crystal light valves (LCLVs) and more specifically to light valves with high photosensitivity and spatial resolution.
2. Description of the Related Art
Liquid crystal light valves (LCLVs) are optical-to optical image transducers that are capable of accepting a low intensity light image and converting it, in real time, into an output image with light from another source. LCLVs have been utilized in a variety of military and commercial large screen graphics applications.
Prior LCLVs, such as the Hughes Liquid Crystal Light Valve described by Rodney D Sterling et al., "Video-Rate Liquid Crystal Light-Valve Using an Amorphous Silicon Photodetector", SID '90 Digest, Paper No. 17A.2, pages 327-329 (1990), use a thick, continuous and uniform layer of amorphous silicon (a-Si) as the photosubstrate with a liquid crystal layer attached to the a-Si photosubstrate. A dielectric mirror is located between the photosubstrate and the liquid crystal layer to reflect the readout beam after it has travelled through the liquid crystal layer and a CdTe light blocking layer is located between the dielectric mirror and the photosubstrate to prevent the readout beam from activating the photosubstrate.
In operation, an AC voltage is applied across the photosubstrate and liquid crystal layer. An input image beam is directed to the input side of the device, while a readout beam impinges on and is reflected from the readout side of the device. The input image beam activates the photosubstrate and reduces the resistivity of the photosubstrate at the area of activation. This, in turn, modulates the voltage across a corresponding area on the liquid crystal layer. Through this effect, the spatial intensity pattern of the input image is converted into a spatial variation in the voltage across the liquid crystal layer which, in turn, results in a spatial variation in the orientation of the liquid crystal molecules. The liquid crystal orientation relative to the readout light polarization at any given location determines the amount of readout light that will be reflected from the light valve at that location. Thus, as the readout beam passes through the liquid crystal layer, it becomes spatially modulated in accordance with the spatial modulation of the liquid crystal orientation.
Ideally, one wants to maximize the amount of voltage that is applied to the liquid crystal layer in response to a resistivity change in the a-Si photoconductive layer. This condition is achieved when the impedances of the liquid crystal layer and the a-Si layer (with no light) are matched. Prior LCLVs achieve this condition by making the a-Si layer very thick (approximately 30 microns). Larger thicknesses are required to match the impedances because the dielectric constant of the a-Si is higher than that of the liquid crystal layer. As the a-Si layer is made thicker, the distance between the electrodes on the input and readout side of the device is increased. This results in a smaller electric field across the liquid crystal and a-Si layers which reduces the response time, spatial resolution, and spectral response of the device. In addition, deposition times of about 30 hours are required to deposit a 30 micron a-Si layer, resulting in increased manufacturing costs.
Another problem is that the thick a-Si layer induces stress on the glass substrate on which the LCLV is typically fabricated on. This stress warps the glass substrate and results in the need for difficult and time consuming polishing procedures to achieve an optically flat surface. The high stress levels also prevent the use of fiber optic substrates which are needed in laser-eye-protection goggles and other display products.
LCLVs also suffer from "charge spreading". In an ideal LCLV, the lateral resistivity of the a-Si layer is high enough to prevent photogenerated charge from migrating in a lateral direction. If charge is allowed to spread laterally in the a-Si layer, then a portion of an input image at one location on the a-Si layer will spread to adjacent locations, resulting in reduced spatial resolution. Prior LCLVs attempt to overcome this problem by increasing the resistivity of the a-Si layer through the use of dopants. A p-type dopant such as boron is added to the a-Si layer to compensate for naturally occurring n-type dopants in the a-Si. This approach is very difficult because one must exactly compensate for the n-type dopants found in the a-Si for this technique to be effective. To complicate matters further, the amount of naturally occurring n-type dopants in the a-Si can change between deposition runs. This difficult counterdoping process results in increased manufacturing costs and lower yields.
The light-blocking CdTe layer found in prior LCLVs is needed to prevent any readout light that leaks through the dielectric mirror from entering the a-Si layer and overwhelming the spatially resolved input image. This CdTe layer has an intrinsic photoconductivity that may also contribute to charge spreading and a corresponding degradation in the spatial resolution of the output image. In addition, since CdTe is a toxic material, additional handling and disposal costs are incurred which add to the overall manufacturing costs of the LCLV.